The use of nuclear quadrupole resonance (NQR) as a means of detecting explosives and other contraband has been recognized for some time. See for example T. Hirshfield et al, J. Molec. Struct. 58, 63 (1980); A. N. Garroway et al, Proc. SPIE 2092, 318 (1993); and A. N. Garroway et al, IEEE Trans. on Geoscience and Remote Sensing 39, 1108 (2001). NQR provides some distinct advantages over other detection methods. NQR requires no external magnet such as required by nuclear magnetic resonance. NQR is sensitive to the compounds of interest, i.e. there is a specificity of the NQR frequencies.
One technique for measuring NQR in a sample is to place the sample within a solenoid coil that surrounds the sample. The coil provides a radio frequency (RF) magnetic field that excites the quadrupole nuclei in the sample and results in their producing their characteristic resonance signals. This is the typical apparatus configuration that might be used for scanning mail, baggage or luggage. There is also a need for a NQR detector that permits detection of NQR signals from a source outside the detector, e.g. a wand detector, that could be passed over persons or containers as is done with existing metal detectors. Problems associated with such detectors using conventional systems are the decrease in detectability with distance from the detector coil, and the associated equipment needed to operate the system.
A detection system can have one or more coils that both transmit and receive, or it can have separate coils that only transmit and only receive. A transmit, or transmit and receive, coil of an NQR detection system provides a radio frequency (RF) magnetic field that excites the quadrupole nuclei in the sample and results in the production of their characteristic resonance signals that the receive, or transmit and receive, coil, i.e. the sensor, detects.
The NQR signals have low intensity and short duration. The transmit, receive, or transmit and receive, coil preferably has a high quality factor (Q). The transmit, receive, or transmit and receive, coil has typically been a copper coil and therefore has a Q of about 102. It can be advantageous to use a transmit, receive, or transmit and receive, coil made of a high temperature superconductor (HTS) rather than copper since the HTS self-resonant coil has a Q of the order of 104–106.
The large Q of the HTS self-resonant coil produces large magnetic field strengths during the RF transmit pulse and does so at lower RF power levels. This dramatically reduces the amount of transmitted power required to produce NQR signals for detection, and thereby reduces the size of the RF power supply sufficiently so that it can be run on portable batteries. The large Q of the HTS self-resonant coil plays a very important role during reception. In view of the low intensity of the NQR signal, it is important to have a signal-to-noise ratio (S/N) as large as possible. The signal-to-noise (S/N) ratio is proportional to the square root of Q so that the use of the HTS self-resonant coil as a sensor results in an increase in S/N by a factor of 10–100 over that of a copper coil sensor.
It is often advantageous to use an array of two or more closely spaced HTS sensors in a NQR detection system. However, coupling of the coils can interfere with system performance. As a result, an object of the present invention is to provide a NQR detection system comprising an array of two or more HTS sensors wherein the HTS sensors have been decoupled.